The present invention is directed to remoistenable hot melt adhesives. In particular, the invention is directed to remoistenable hot melt adhesives characterized by an outstanding balance of non-block characteristics under heat and high humidity conditions combined with excellent bond strength.
Hot melt adhesives achieve a solid state and resultant cohesive strength by cooling, as opposed to other types of adhesives which achieve the solid state through evaporation or removal of solvents, or by polymerization. At room temperature, a hot melt applicable adhesive is a thermoplastic, non-volatile solid material. When the hot melt adhesive is heated, it liquifies; after removal of heat, it solidifies on cooling. Hot melt adhesives are usually used for reasons such as process speed, streamlining of process equipment by elimination of driers and other auxiliary equipment or the like. To be water-moistenable, an adhesive composition must be activatable by moistening with water.
A water-moistenable hot melt applicable adhesive composition is a bonding agent which may be applied to one surface utilizing a hot melt technique and another surface using a water-moistening technique, so that the two surfaces are bonded together by means of the composition. By way of example, such a composition may be applied by the hot melt technique as gumming for labels, sealing tapes and envelope flaps at a point of manufacture where the intricate equipment required for hot melt application is available, the labels, sealing tape and envelope flaps, ultimately being applied by the consumer to other surfaces using the much simpler water-moistening technique, e.g., by moistening the adhesive coating prior to application to the other surface. The hot melt application is utilized where the required equipment is available because it is a rapid and economical technique; the water-moistening technique is utilized precisely because it doesn't require such elaborate equipment.
In attempting to provide a single adhesive composition which is to be applied to one surface using a hot melt technique and another surface using a water-moistening technique, considerations arise which are not present in the provision of an adhesive composition to be applied by either technique alone. For example, most water-moistenable adhesives do not possess the requisite thermoplastic properties permitting hot melt application of desirably smooth glossy coats thereof. Similarly, most hot melt applicable thermoplastic adhesives are water insensitive and cannot be readily tackified by water-moistening. Moreover, such adhesive compositions must obviously be heat-tackifiable to be applied by hot-melt techniques and also moisture-tackifiable to form bonds by water-moistening techniques. On the other hand, once the composition has been hot-melt applied to a substrate, it must be able to resist elevated temperature conditions likely to be found in its environment without again becoming tackifiable or undergoing other undesirable changes, such as discoloration (darkening), over a prolonged period of time. Similarly, it must be able to withstand the various humid conditions likely to be found in its environment without becoming tacky until it is actually moistened immediately prior to use. Consider, for example, the common phenomenon of postal envelopes with water-moistenable gumming on the flaps which become prematurely sealed on humid summer days. The same problem occurs on a larger scale within the adhesive manufacturing plant when an adhesively coated substrate carrying the hot-melt applied composition is formed into a layered roll or sheet-on-sheet storage configuration and the adhesive of one layer or sheet starts to adhere to the substrate of the layer or sheet underneath. The tendency of the adhesive to undergo premature sealing under humid conditions is measured in terms of the humidity blocking resistance at a given relative humidity (under a given load at a given temperature, for a given period of time). The blocking resistance is normally measured in terms of the percentage tear occurring, with high levels of blocking producing high levels of tear when the adhesive-bearing substrate is removed from the opposed surface.